Electronic devices such as electronic book reader devices (“eBook reader devices”), cellular telephones, portable media players, desktop computers, laptops, tablet computers, netbooks, personal digital assistants, and the like, rely on electrical power to function.
Within these electronic devices, several components utilize significant amounts of power during operation, including the processor(s) and peripheral devices. These peripherals include external memory interfaces (EMIs), Universal Serial Bus (USB) controllers, image processing units (IPUs), and so forth. These peripherals may reside on the same “chip” or die as the processor, on another die, or a combination of same die and another die.
A peripheral device that is not actively in use but idling continues to consume significant quantities of power and may prevent a central processor from entering a lower power mode. This idling wastes energy and increases the amount of heat dissipated by the electronic device. Reducing power consumption increases the usable time for a portable device operating from a battery. Reducing power consumption also reduces the heat dissipated by the electronic device, allowing it to operate at a cooler temperature and thus increasing the life of the equipment and simplifying the cooling design.
Various schemes have been put forth to reduce power consumption in portable consumer devices by placing the processor of the device and the peripherals into lower power modes. Typically, this involves turning off unused portions of the circuit and/or reducing clock speed of the microprocessor.
However, the techniques used to enter the lower power modes may introduce unacceptable latencies to execution of commands upon resumption of normal activity, particularly with regards to peripherals. Additionally, recurrent timer interrupts to query peripherals often prevent a device from entering a lower power mode, or from remaining in the lower power mode long enough to realize a meaningful reduction in power consumption.